1. Field of the Invention
The present invention relates to a scanning optical apparatus and an image forming apparatus using the same. The present invention can suitably be applied to image forming apparatuses such as laser beam printers (LBP) using an electronic photography process, digital copying machines, and multi-function printers.
2. Description of the Related Art
Conventionally, in scanning optical apparatuses such as laser beam printers (LBP) and digital copying machines, a light beam that has been optically modulated in accordance with an image signal and emitted from a light source unit is converted into a collimated light beam by a collimator lens. Then, the light beam that has been made collimated is focused by a cylindrical lens into a linear image formed on a deflecting surface of a light deflector constituted by a rotary multi-face mirror (or polygon mirror) for the purpose of image tilt compensation. Then, the light beam deflectively scanned by the deflecting surface of the light deflector is focused by an imaging optical system having an fθ characteristic into a spot on the surface of a photosensitive recording medium (or photosensitive drum) to optically scan that surface, thereby recording an image.
Heretofore, various tandem type scanning optical apparatuses have been developed (see Japanese Patent Application Laid-Open No. 2003-222812). In the tandem type scanning optical apparatuses, a plurality of divergent light beams emitted from a plurality of light source units are narrowed by a plurality of corresponding aperture stops, converted into collimated light beams by a plurality of collimator lenses, and incident on a plurality of cylindrical lenses. The plurality of light beams emergent from the plurality of cylindrical lenses are incident on different deflecting surfaces of an optical deflector (or polygon mirror) serving as a deflection unit to form linear images, and deflectively scanned in different directions respectively. The light beams thus deflectively scanned are guided respectively onto different surfaces to be scanned (or photosensitive drum surfaces) by different imaging optical units. Image recording is performed in this way.
Heretofore, there has been developed various scanning optical apparatuses in which changes in the imaging performance caused by environmental changes, specifically temperature changes (temperature compensation characteristics) and changes in the imaging performance caused by changes in the wavelength of laser light (wavelength characteristics) are made small (see Japanese Patent Application Laid-Open No. 2002-287062).
In the conventional tandem type scanning optical apparatuses described above, in order to facilitate ease of assembly, the plurality of cylindrical lenses are made up of a plurality of optical elements arranged along the main scanning direction, which are integrally manufactured by plastic molding. Japanese Patent Application Laid-Open No. 2002-228812 discloses a case in which cylindrical lenses, which are commonly provided for the respective light sources separately, are made integral, whereby, according to the description in this patent document, reduction in the entire size of the apparatus can be achieved.
In recent years, in order to achieve further size reduction and to facilitate ease of assembly, it has been required in the tandem type scanning optical apparatus to make integral the collimator lenses that convert the conditions of light beams emitted from light source units into other conditions. However, it has been considered difficult to make the collimator lenses integral for the following reasons.
(1-1) It is necessary to adjust the position of the collimator lenses relative to the corresponding light emitting portions (or light emitting points) of a plurality of light source units with respect to the direction of the optical axis and the directions perpendicular to the optical axis. (This adjustment will be hereinafter referred to as the “laser adjustment”.)
(1-2) In cases where the light source unit is adjusted in the laser adjustment, it is necessary to once press-fit the light source unit into an intermediate member and thereafter adjust it, because the shape of the laser package is complex.
(1-3) Closeness of the plurality of light source units to each other will lead to physical interference of adjusting mechanisms and jigs that hold them.
On the other hand, to combine parts, it is desirable from the viewpoint of productivity to manufacture them by plastic molding. In cases where plastic molding is employed, there arises a problem that a change in the environmental condition (e.g. temperature) will cause a change in the refractive index in the plastic, leading to a displacement of the focus position after beam conversion, in addition to the above described problems (1-1) to (1-3). Especially in the case of a multi-beam light source having a plurality of light emitting portions, one problem is that there is a difference in the wavelength between the light emitting portions by manufacturing tolerance, which makes it difficult to employ conventional temperature compensation using a refracting surface and diffracting surface and utilizing a change in the wavelength caused by a temperature rise.
On the other hand, if collimator lenses made of a plastic are realized, anamorphic surfaces can easily be formed, and therefore it is possible to integrate the functions of collimator lens and cylindrical lens (which are separate parts in the conventional apparatuses) into one part. Japanese Patent Application Laid-Open No. 2002-287062 discloses, in FIG. 2 thereof, temperature compensation for an optical element having two transmitting surfaces and at least one reflecting surface in a laser scanning apparatus. However, this patent document does not suggest any solution to the problem of wavelength characteristics in the case where multi-beam light source is employed nor disclose an optical configuration that is designed in such a way as to facilitate ease of assembly when applied to a tandem type scanning optical apparatus.